1966 Pontiac GTO Disc Brake Fix

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The Combo

Rick Bogoff’s show-quality ’66 Pontiac GTO has a fully built H.O. Enterprises 10.3:1 455 equipped with Edelbrock aluminum heads, Comp Cams’ biggest Xtreme Energy XE294H hydraulic flat-tappet cam (250/256-degrees duration at 0.050), and a Demon 750-cfm vacuum-secondary carb atop an Edelbrock Performer RPM intake. A TH400 automatic transfers power back to a 3.73:1-geared rearend. The car hugs the road with a full Air Ride suspension and rolls on Nitto 225/50R17 tires up front and 275/50R17 tires out back. With its huge rotors and calipers, Stainless Steel Brakes’ four-wheel power discs should have stopped the car on a dime. But they weren’t living up to their potential.

1/19“It finally stops like four-wheel disc brakes should,” car owner Rick Bogoff says, now all smiles after his brakes were no longer broke.

The Problem

The GTO’s pedal was firm—hard as a rock after 3 to 4 inches of brake-pedal movement—but the brakes simply wouldn’t lock up. In an attempt to fix the problem, Bogoff moved the brake pedal pushrod to the top hole in the pedal arm for more pedal advantage, switched out the supplied aftermarket 1.125-inch-bore master cylinder for a 1-inch bore unit to increase pressure, installed an aftermarket vacuum canister, replaced the vacuum power booster’s inlet check valve with a new valve, and tried multiple settings on an adjustable proportioning valve plumbed into the rear-brake system.

2/19The show-quality ’66 GTO had custom everything, inside and out, but even with a firm pedal, the brakes wouldn’t lock up.

None of this helped, so Bogoff trailered the Wheeling, Illinois–based car over to southern Wisconsin’s shop of last resort, Norm Brandes’ Westech Automotive. When the car was unloaded, the brakes couldn’t even stop it after it rolled down the trailer ramps.

As-received hydraulic pressures at the caliper were no higher than 800 psi. Idle vacuum was only 6 to 8.5 inHg, although the power-booster did retain what vacuum it had after engine shutdown. The brake system had only a few hundred miles on it, but the rotors were already badly glazed.

3/19Was it the vacuum booster, the master cylinder, the calipers, the rotors, the engine’s state of tune, or all of the above?

The Fix: Pads and Discs

In a brake system, friction is generated by both forcing the brake pads mechanically against the spinning rotor (abrasive friction), and by transferring a thin layer of brake-pad material to the rotor face as temperatures rise (adherent friction). You need to get enough heat into the brake system to get this transfer process started. Because the car wouldn’t stop, Bogoff never drove fast enough to generate the needed heat. “It was like trying to apply brake pads against a pane of glass,” Brandes explains.

Westech turned the rotors and installed new, high-quality ceramic brake pads. The new pads were then bedded to the rotors. “Bedding” is a systematic process of repeatedly heating the brakes to the pad compound’s designed temperature range under controlled braking in order to generate the necessary transfer layer. Ninety-nine percent of us slap on a new set of pads and motor on down the road, but Brandes says proper bedding can improve brake performance by 30 to 40 percent.

By accelerating to 60 mph and quickly backing off the throttle, the engine momentarily sucked 18 inches of vacuum, enough for up to two hard pedal applications per session—sufficient to begin bedding in the rotors but clearly not adequate for stop-and-go driving.

The Fix: Tune-Up

With the pads and rotors stabilized, Brandes next plumbed a hose from his V10 Dodge shop truck to the GTO’s vacuum booster. Brake pressures generated at idle jumped way up, indicating no apparent brake-component parts malfunction. That left insufficient engine vacuum production as the culprit.

5/19A number of things needed attention, but adding an electric vacuum pump was the major factor in getting the brakes right.

The Demon carb lacked adjustable idle air bleeds needed to achieve proper idle-mixture quality with a big cam. The rarely driven car’s engine had fallen out of tune and was running rich, further exacerbating the problem. Given its adjustable bleeds, adding a Quick Fuel primary metering block onto the Demon would have stabilized the idle, but for superior performance throughout the entire envelope, the owner volunteered to step all the way up to a new Quick Fuel 830-cfm mechanical-secondary carb. Brandes also fine-tuned the ignition curve and threw in fresh spark plugs. The new carb and tune gave the car about 1-inch of additional idle vacuum, but it was still less than the 11.5 to 12 inches Brandes likes to see with an auxiliary vacuum canister.

The Fix: Vacuum Pump

Plumbing in an electric vacuum pump finally did the trick. To address owner concerns that the pump was too noisy at engine crank, Westech added an oil-pressure switch not included in the pump kit to prevent the pump from running until the engine is going. With the existing 1-inch-bore dual master cylinder; 9-inch-od, single-diaphragm booster; and vacuum reserve canister—plus the new vacuum pump sucking 22 inHg—caliper pressures hit 1,300 psi up front and 1,000 rear at 150 psi input pedal-pressure with the prop-valve zeroed out. Not bad, but we’ve seen more than 1,500 psi on properly set up GM muscle car–era power-disc swaps. Brandes was deciding what to do next when fate intervened: The 9-inch, single-diaphragm booster, now seeing 22 inches of vacuum, packed it in and sprung an internal leak.

6/19No longer all-go, no-whoa: With its “high-caliper” brakes now fully functional, the GTO is once again fun to drive on a daily basis.

The Fix: Dual Vacuum Booster

That offered a chance to put in a better booster. There wasn’t room for a wider unit, due to the Poncho V8’s tall valve covers. And a chrome-plated replacement would be needed for the show-quality GTO. Tuff Stuff came to the rescue, supplying a longer, higher-capacity, 9-inch dual-diaphragm chromed booster.

7/19As received, the rotors were scoured and glazed. There was insufficient hydraulic pressure at the calipers, so the pads and rotors never got hot enough to transfer the necessary thin layer of brake-pad material to the rotor face. Think of it as a cylinder bore in which the piston rings never seated.

But the booster didn’t want to cleanly marry up to the existing 1-inch master. Brandes was able to jury-rig it with spacers and shims for test purposes. Man, did it work, boosting the system as high as 1,800 psi front/1,100 psi rear at 22 inches. It also developed more pressure at lower vacuum. The brakes were now so sensitive with the 1-inch master that under moderate brake application typical of most street driving, there was actually a lack of pedal modulation. An “all or nothing” pedal might be a good fit on a race car, but it was too herky-jerky for a daily driver.

The Fix: 1 1⁄8 Master

With the existing 1-inch master not in a mating mood, Tuff Stuff’s recommended 1.125-inch-bore unit was ordered and bolted up to the booster. It greatly improved partial pedal modulation and feel. Normally you’d expect a slight pressure loss with a larger-bore cylinder, but pressures in this case were still within 100 psi or so of those generated by the 1-inch unit. Apparently, the vacuum-assist overcame any bore-size difference.

8/19The rotors were turned and new ceramic pads were installed. See the lighter, virgin area below the demarcation line just out from the rotor hub? Compare it with the darker, uniform, gray-color outboard from that line, which indicates that (thanks to careful bedding-in with sufficient pressure), the proper material-transfer process has been initiated.

Results

Under hard braking, the GTO now stops so hard that a passenger not belted in nearly dented the windshield, yet around town the new booster and master cylinder combo yields great pedal feel and modulation. As Brandes puts it, “It really sold me on the dual-diaphragm concept. Even a petite lady could drive this car now.” The limiting factor on stopping performance is no longer the brakes but the tires’ limit of adhesion.

Electrical side: With the key circuit "On," a wire connected to a spare fuse-box terminal supplies power to the vacuum pump as well as to the relay's "control" circuit at Terminal 2. When oil pressure exceeds 5 psi at the pressure switch on the oil-filter housing and vacuum drops below 17 inHg at the vacuum switch spliced into the pump's induction-side hose, the control circuit closes. Current flows out from Relay Terminal 1 through the switches to Ground. Magnetic induction then closes the relay's "power" side, and current flows from the vacuum pump to Relay Terminal 5, through the Relay, and out Terminal 3 to Ground, energizing the pump until vacuum again exceeds 22 inHg. The vacuum switch opens, interrupting flow through the control circuit, in turn de-energizing the relay's power-side circuit to turn off the pump.

9/19

Vacuum side: When the pump runs, its induction-side hose sucks air out of the booster and vacuum canister, through the vacuum switch, and back to the pump. The pump's exhaust side is plumbed to an engine-manifold vacuum source; if the pump fails, the engine can still supply some vacuum to the system.

This data shows the pressures produced at the calipers under differing amounts of vacuum and brake-pedal force at various modification stages. Unless otherwise noted below, vacuum readings were obtained under the following conditions:

Pressure Tests

10/19There was room for a longer, dual-diaphragm booster of the same diameter. Brandes installed Tuff Stuff’s chrome 9-inch dual-diaphragm unit (right, versus original 9-inch single, left). Unlike the integral mounting brackets used on most stockers, aftermarket boosters have bolt-on brackets, allowing the same unit to fit a wide variety of firewalls.

With 9-inch Dual-Diaphragm Booster: As previous, but with new carb and new booster; tune-up was factored out by adjusting mixture and idle speed to replicate previous 6.5 and 8 inHg vacuum check points.

Vacuum

Pedal

Front Caliper

Rear Caliper

6.5 inHg

50 psi

500 psi

600 psi

6.5 inHg

100 psi

700 psi

725 psi

6.5 inHg

150 psi

900 psi

800 psi

8 inHg

50 psi

650 psi

550 psi

8 inHg

100 psi

1,600 psi

700 psi

8 inHg

150 psi

1,700 psi

850 psi

22 inHg

50 psi

1,000 psi

800 psi

22 inHg

100 psi

1,500 psi

1,000 psi

22 inHg

150 psi

1,800 psi

1,100 psi

With 1.125-inch-Bore Master Cylinder: As previous, but with new larger-bore master cylinder.

Vacuum

Pedal

Front Caliper

Rear Caliper

22 inHg

50 psi

1,100 psi

900 psi

22 inHg

100 psi

1,300 psi

1,000 psi

22 inHg

150 psi

1,900 psi

1,100 psi

Lessons Learned

Four-wheel, power-disc-brake setups need at least a 9-inch, dual-diaphragm power booster--even bigger if there's room. If you're running a big cam with power brakes, add a vacuum pump.